It is well established in mammals that chronic, long-term elevations in sex steroids are associated with improvements in spatial cognition. It is less clear the extent to which short to medium term elevations in sex steroids improve spatial cognition and change hippocampal morphology, particularly in birds. The avian hippocampus expresses both androgen receptors (AR) and oestrogen receptor alpha (ERα) and high levels of the enzyme aromatase that converts testosterone to oestrogen. I began by comparing spatial cognition, hippocampal sex steroid receptor and aromatase expression between males and females. There were no differences in spatial or visual cognition or in hippocampal sex steroid receptor expression between the sexes, although hippocampal aromatase mRNA expression was higher in males. I then addressed the effects of acute and medium-term sex steroid treatment on spatial cognition and hippocampal aromatase and sex steroid receptor expression. A single treatment of testosterone 30 minutes or four hours prior to cognitive testing improved spatial performance. Additionally, when testosterone and oestrogen were given daily for five days spatial cognition in both sexes was improved. The testosterone-induced improvement was blocked when testosterone was administered in conjunction with the aromatase inhibitor fadrozole but not when administered with saline. These findings suggest that spatial cognition is improved by an oestrogenic effect. Thirty minutes following acute testosterone treatment, plasma testosterone levels, hippocampal AR and ERα mRNA expression all increased. Five days of oestrogen treatment increased plasma oestrogen levels, hippocampal ERα mRNA and Nmethyl- D-aspartate (NMDA) receptor levels in males and females; all were positively correlated with enhanced spatial cognition on day five of treatment. Finally, I determined which genes were differentially expressed as a result of five days of oestrogen treatment. Nineteen genes, identified as being involved in learning and memory were differentially expressed in the hippocampus, eleven of which were up-regulated and eight were down-regulated. Taken together these results demonstrate that oestrogen can improve spatial cognition in birds. It is plausible that oestrogen acts to improve spatial memory in the hippocampus through upregulation of genes that control neurotransmitter release, reuptake and receptor levels.

<p>Schizophrenia is a complex and debilitating psychiatric illness characterized by positive symptoms like hallucinations and delusions and negative symptoms like blunting of affect, avolition, and poverty of thought. This constellation of symptoms is hypothesized to result from dopaminergic dysfunction, glutamatergic dysfunction, and dysfunctional stress-reactivity. Prior to the onset of schizophrenia there is a prodromal period when individuals begin to experience sub-clinical symptoms and decreased functioning. This period is important to study not only to help elucidate biologic mechanisms of the illness but also to potentially alter the course of the illness through early treatment. The difficulty of studying this period lies in its recognizing it prospectively. To address this researchers have begun to study the at-risk mental state, a state that is associated with a high but not inevitable risk of conversion to psychosis. The studies described in this dissertation are aimed at a neurofunctional characterization of the at-risk mental state in three primary domains: reward-anticipation, hippocampus-dependent learning, and stress-reactivity. Individuals at-risk for psychosis and age-matched healthy volunteers underwent functional magnetic resonance imaging while performing tasks targeting these domains. In the reward-anticipation task, at-risk individuals showed decreased ventral tegmental area (VTA) and dorsolateral prefrontal cortex (DLPFC) responses to reward anticipation. In the hippocampus-dependent learning task, at-risk individuals showed deficits in hippocampus-dependent memory, decreased VTA engagement, and increased DLPFC activation during learning of associations between items. In the stress-reactivity task, at-risk individuals showed increased activation in the bed nucleus of the stria terminalis/basal forebrain (BNST), anterior cingulate cortex (ACC), and medial prefrontal cortex (mPFC) in response to neutral faces. Collectively, these experiments show that neurofunctional deficits in reward-anticipation, hippocampus-dependent learning, and stress-reactivity are present in the putative prodrome, prior to the onset of psychosis. Regions implicated are those that would be expected based on current models of schizophrenia and neurofunctional studies in those with frank psychosis.</p> / Dissertation

Chronic ethanol (EtOH) exposure produces neuroadaptations within the NMDA receptor system and alterations in HPA axis functioning that contribute to neurodegeneration during ethanol withdrawal (EWD). Chronic EtOH exposure and EWD, as well as corticosteroids, also promote increased synthesis and release of polyamines, which allosterically potentiate NMDA receptor open-channel time at the NR2B subunit. The current studies investigated effects of 10 day EtOH and corticosterone (CORT) co-exposure on toxicity during EWD in rat organotypic hippocampal slice cultures, and alterations in function and/or density of the NR2B subunit of the NMDA receptor that may mediate CORT-potentiation of toxicity during EWD. We hypothesized that toxicity during withdrawal following EtOH and CORT co-exposure would be greatest in the CA1 region due to increased NMDA NR2B receptor abundance and/or function. Cultures were exposed to CORT (0.01–1 μM) during 10 day EtOH exposure (50 mM) and 1 day EWD. Additional EtOH-naïve cultures were exposed to CORT for 11 days. Propidium iodide (PI) was used to measure toxicity in the CA1, CA3, and DG hippocampal regions. In EtOH-naïve cultures, 11 day exposure to CORT (0.01 – 1 μM) produced modest toxicity and in all regions. Exposure to CORT during EtOH exposure/EWD potentiated CORT-toxicity at all concentrations in the CA1 region. Ifenprodil, an NR2B polyamine site antagonist, significantly reduced toxicity from EtOH and CORT (0.1 μM) co-exposure during withdrawal. Immunohistochemistry and Western blot analyses were conducted for measurement of NR2B immunoreactivity in organotypic cultures, and autoradiography studies were conducted for measurement of polyamine-sensitive NR2B subunits with [3H]ifenprodil. Consistent increases in NR2B subunit protein were not detected with use of any methodology. Additional studies exposed cultures to a membrane impermeable form of CORT (BSA-conjugated CORT; 0.1 μM) with or without EtOH exposure and withdrawal. BSA-CORT exposure did not produce toxicity in any hippocampal region, suggesting that CORT toxicity was not mediated by membrane bound substrates. These data suggest that CORT and EtOH co-exposure result in increased function of polyamine-sensitive NR2B subunits, but this toxicity does not appear dependent on the number of hippocampal NMDA NR2B subunits.

In the field of imaging genetics, brain function and structure are used as intermediate phenotypes between genes and cognition/diseases to validate and extend findings from behavioral genetics. In this thesis, three of the strongest candidate genes for episodic memory, KIBRA, BDNF, and APOE, were examined in relation to memory performance and hippocampal/parahippocampal fMRI blood-oxygen level-dependent (BOLD) signal. A common T allele in the KIBRA gene was previously associated with superior memory, and increased hippocampal activation was observed in noncarriers of the T allele which was interpreted as reflecting compensatory recruitment. The results from the first study revealed that both memory performance and hippocampal activation at retrieval was higher in T allele carriers (study I). The BDNF 66Met and APOE ε4 alleles have previously been associated with poorer memory performance, but their relation to brain activation has been inconsistent with reports of both increased and decreased regional brain activation relative to noncarriers. Here, decreased hippocampal/parahippocampal activation was observed in carriers of BDNF 66Met (study II) as well as APOE ε4 (study III) during memory encoding. In addition, there was an additive gene-gene effect of APOE and BDNF on hippocampal and parahippocampal activation (study III). Collectively, the results from these studies on KIBRA, BDNF, and APOE converge on higher medial temporal lobe activation for carriers of a high-memory associated allele, relative to carriers of a low-memory associated allele. In addition, the observed additive effect of APOE and BDNF demonstrate that a larger amount of variance in BOLD signal change can be explained by considering the combined effect of more than one genetic polymorphism. These imaging genetics findings support and extend previous knowledge from behavioral genetics on the role of these memory-related genes.

No description available.

Verbal memory (VM) has been shown to be impacted in brain tumor (BT) survivors, but the nature of VM problems and underlying neuropathology are poorly understood and a long-term outlook is lacking. Our study examined hippocampus volume (HV) and VM in adult survivors of pediatric BT (n=32) and controls (n=48). Results indicate that disruption to a maturing brain in childhood is detectable 17 years (mean) after diagnosis, as HV is significantly lower in survivors compared to controls. Analysis of the VM scores shows that survivors have significantly lower overall immediate recall compared to controls, but learning slope, retention, and recognition are not different across the groups. Survivors’ memory profile indicates that auditory attention and retrieval difficulties could be contributing to their lower immediate recall. For survivors, HV is significantly correlated with delayed free recall but not with other VM indices. Implications of these findings are discussed.

The nervous system, including the brain, is a complex network with billions of complex neurons. Ion channels mediate the electrical signals that neurons use to integrate input and produce appropriate output, and could thus be thought of as key instruments in the neuronal orchestra. In the field of neuroscience we are not only curious about how our brains work, but also strive to characterize and develop treatments for neural disorders, in which the neuronal harmony is distorted. By modulating ion channel activity (pharmacologically or otherwise) it might be possible to effectively restore neuronal harmony in patients with various types of neural (including channelopathic) disorders. However, this exciting strategy is impeded by the gaps in our understanding of ion channels and neurons, so more research is required. Thus, the aim of this thesis is to improve the understanding of how specific ion channel types contribute to shaping neuronal dynamics, and in particular, neuronal integration, excitability and memory. For this purpose I have used computational modeling, an approach which has recently emerged as an excellent tool for understanding dynamically complex neurophysiological phenomena. In the first of two projects leading to this thesis, I studied how neurons in the brain, and in particular their dendritic structures, are able to integrate synaptic inputs arriving at low frequencies, in a behaviorally relevant range of ~8 Hz. Based on recent experimental data on synaptic transient receptor potential channels (TRPC), metabotropic glutamate receptor (mGluR) dynamics and glutamate decay times, I developed a novel model of the ion channel current ITRPC, the importance of which is clear but largely neglected due to an insufficient understanding of its activation mechanisms. We found that ITRPC, which is activated both synaptically (via mGluR) and intrinsically (via Ca2+) and has a long decay time constant (τdecay), is better suited than the classical rapidly decaying currents (IAMPA and INMDA) in supporting low-frequency temporal summation. It was further concluded that τdecay varies with stimulus duration and frequency, is linearly dependent on the maximal glutamate concentration, and might require a pair-pulse protocol to be properly assessed. In a follow-up study I investigated small-amplitude (a few mV) long-lasting (a few seconds) depolarizations in pyramidal neurons of the hippocampal cortex, a brain region important for memory and spatial navigation. In addition to confirming a previous hypothesis that these depolarizations involve an interplay of ITRPC and voltage-gated calcium channels, I showed that they are generated in distal dendrites, are intrinsically stable to weak excitatory and inhibitory synaptic input, and require spatial and temporal summation to occur. I further concluded that the existence of multiple stable states cannot be ruled out, and that, in spite of their small somatic amplitudes, these depolarizations may strongly modulate the probability of action potential generation. In the second project I studied the axonal mechanisms of unmyelinated peripheral (cutaneous) pain-sensing neurons (referred to as C-fiber nociceptors), which are involved in chronic pain. To my knowledge, the C-fiber model we developed for this purpose is unique in at least three ways, since it is multicompartmental, tuned from human microneurography (in vivo) data, and since it includes several biologically realistic ion channels, Na+/K+ concentration dynamics, a Na-K-pump, morphology and temperature dependence. Based on simulations aimed at elucidating the mechanisms underlying two clinically relevant phenomena, activity-dependent slowing (ADS) and recovery cycles (RC), we found an unexpected support for the involvement of intracellular Na+ in ADS and extracellular K+ in RC. We also found that the two major Na+ channels (NaV1.7 and NaV1.8) have opposite effects on RC. Furthermore, I showed that the differences between mechano-sensitive and mechano-insensitive C-fiber types might reside in differing ion channel densities. To conclude, the work of this thesis provides key insights into neuronal mechanisms with relevance for memory, pain and neural disorders, and at the same time demonstrates the advantage of using computational modeling as a tool for understanding and discovering fundamental properties of central and peripheral neurons. / <p>QC 20120914</p>

No description available.

Despite their acknowledged efficacy, currently available antidepressants still demonstrate undesirable side effects, shortfalls in effectiveness and a delayed onset of action. All these agents act via monoaminergic mechanisms, although recent studies have begun to note the potential role of the cholinergic system as well as the amino acid pathways in affective isorders. It has been suggested that glutamate NMDA receptor activation may be involved in hippocampal degeneration seen in patients with depression, as well as contributing as a molecular target for the antidepressant action of known antidepressant drugs. Glutamate either separately or via the release of nitric oxide, regulates the release of various transmitters in the brain critical for affective state, e.g. monoamines (noradrenaline, dopamine), indoleamines (5HT), y-aminobutyric acid (GABA) and acetylcholine. The aim of this study was to investigate N-methyl-D-aspartate (I\IMDA) and muscarinic M1 receptor characteristics and also GABA and acetylcholine levels in a genetic animal model of depression, the Flinders Sensitive Line (FSL) rat, with respect to its control, viz. Flinders Resistant Line (FRL) rat, thereby establishing a possible role for the amino acid and cholinergic pathways in the hippocampus and frontal cortex, two brain areas implicated in depression. In addition, anxietylike behaviours were assessed using the open field and social interaction tests. A sensitive liquid chromatography tandem mass spectrometer (LC/MS/MS) method was used in the quantification of acetylcholine as well as high performance liquid chromatography with electrochemical detection (HPLG-EGD) for the quantification of GABA in the above-mentioned brain areas of FSL and FRL rats. NMDA and muscarinic M1 receptor characteristics were expressed in terms of receptor denSity (Bmax) and affinity (Kd) values and were performed using [3H]-MK801 (27.5 Gi/mmol) and quinuclidinyl benzilate (52.0 Gilmmol) for NMDA and M1 receptors, respectively. In addition, to provide evidence for face validity, behavioural assessments were routinely performed using the open field test and social interaction test. Significantly elevated levels of acetylcholine were found in the frontal cortex but with significantly reduced levels in the hippocampus of FSL rats. Cortical and hippocampal muscarinic receptor binding characteristics remained unchanged, while no differences with regard to GABA levels and NMDA receptor binding characteristics were noted in these brain areas. In concordance with studies from the literature, aversive and locomotor behaviour as measured in the open field test, provided evidence of anxiogenic behaviour in the FSL rat, evinced by significantly less social interaction than their FRL counterparts. In addition, evidence for a lack in general activity of the FSL rat in the open field was also noted. Our data therefore suggest the presence of a cholinergic dysfunction in both the frontal cortex and hippocampus of the FSL rat, although this is not accompanied by simultaneous changes in muscarinic M1 receptor binding in key limbic brain regions. Although increased cholinergic drive is a recognised characteristic of FSL rats and is representative of the model's' construct validity, we suggest that the depressive phenotype of these animals is not related to altered cholinergic activity in a single brain region, but instead involves various limbic brain regions, possibly being more dependent on opposing cholinergic deficits in the cortex and hippocampus. / Thesis (M.Sc. (Pharmacology)--North-West University, Potchefstroom Campus, 2009.

<p>Estradiol modulates the use of spatial navigation strategies in female rats. The presence of circulating estradiol enhances learning on tasks that require the use of a hippocampus-dependent place strategy and impairs learning on tasks that require the use of a dorsal striatum-dependent response strategy. When either strategy may be used successfully, estradiol biases females to use a place strategy. While this behavioral effect has been well-described in the young adult female rat, little is known about the mechanisms in the brain that underlie it or how it changes across age. The experiments in this dissertation examined how age, previous experience, and hormonal condition affect the ability of estradiol to modulate learning during explicit training of place and response tasks, as well as navigation strategy use during ambiguous navigation tasks. Age highly influenced the ability of estradiol to influence strategy use. While female rats could use place and response strategies to navigate by postnatal day (PD) 21, estradiol did not bias them to use a response strategy until PD26, just before puberty. In adulthood, previous navigation experience and estradiol interacted to influence navigation strategy use on a series of experiences to an ambiguous navigation task. And, estradiol impaired learning during explicit response training but did not affect place learning. In middle age, estradiol further impaired response learning but still did not affect place learning. Long-term hormone deprivation, however, was detrimental to acquisition of a place task but did not affect response learning. These experiments also examined the effects of estradiol on activity, plasticity, and reliability of neuronal ensembles in several subregions of the hippocampus and striatum during spatial navigation using cellular and molecular techniques that take advantage of the kinetics of the immediate-early genes c-fos and Arc. Increased activation and plasticity during active exploration across several subregions of the hippocampus and striatum reflected similar inputs to these neural systems and similar effects of exploration. However, estradiol modulated the plasticity and reliability of neuronal ensembles in the hippocampus and striatum specifically during goal-directed spatial navigation. Estradiol increased plasticity in CA1 of all behaviorally-trained rats, but only place strategy users displayed high reliability in this plasticity across training and probe trials on a navigation task. Estradiol prevented increase in plasticity and reliability in the dorsolateral striatum displayed by low estradiol response strategy users. These experiments reveal how several factors, including age, influence estradiol's modulation of spatial navigation strategy use and suggest functional mechanisms by which this modulation occurs.</p> / Dissertation